This invention relates to an in-focus sensing apparatus of optical equipment, such as a microscope, that allows observation in a wide range from low to high magnifications.
In connection with an apparatus that senses the optical images before and after the image formation plane with the image sensor, and adjusts the distance between the objective optical system and specimen so that the difference between the electric signals corresponding to the light intensities of those two optical images may be kept at a specified value, Published Examined Japanese Patent Application No. 61-60413 has disclosed a focal-point adjusting apparatus provided with means for varying the optical path between the image formation plane and image sensor as the magnification of the objective optical system is changed.
Additionally, Published Unexamined Japanese Patent Application No. 63-78113 has disclosed an automatic focal-point adjusting mechanism which divides the measuring light beam going from the subject to the line sensor into a plurality of light beams, shifts the light formation positions of the divided light beams forward and backward along the optical axis, performs the in-focus operation by comparing the signals of two optical images, and enables a high regular-transmission transparent member whose refractive index differs from that of air to withdraw into one of the divided light beams during the time from when the measuring light beam has been divided until the light beams reach the line sensor, in the optical path of the measuring light beam.
The method of projecting the optical images before and after the expected focal plane onto the image sensor and comparing the signals of two optical images to sense the in-focus point has a disadvantage in that use of a high-magnification objective makes the difference between the signals of two optical images before and after the expected focal plane so small that the in-focus point cannot be sensed. To solve this problem, a method has been proposed which is capable of changing the difference in the optical path between the expected focal plane and image sensor according to the change of the magnification of the optical system, such as the change of objectives, and which makes the optical path difference greater in the case of high magnification, as disclosed in Published Examined Japanese Patent Application No. 61-60413. This method, however, has disadvantages in that the large optical path difference necessary for high magnification makes the optical path space larger, consequently the apparatus greater, and that an arrangement to change the optical path difference is needed.
Another disadvantage is that at high magnification, since the optical image is divided into two optical images in spite of the amount of light of the original optical image is very small, the amount of light of the resulting images is even smaller, which makes the sensing of the in-focus point difficult.